mkfs.xfs: don't increase agblocks past maximum

mkfs.xfs: don't increase agblocks past maximum

RH QA discovered this bug:

Steps to Reproduce:
1. Create 4 TB - 1 B partition
    dd if=/dev/zero of=x.img bs=1 count=0 seek=4398046511103
2. Create xfs fs with 512 B block size on the partition
    mkfs.xfs -b size=512 xfs.img

Actual results:
Agsize is computed incorrectly resulting in fs creation fail:
agsize (2147483648b) too big, maximum is 2147483647 blocks

This is due to the "rounding up" at the very end of the calculations;
there may be other places to alleviate the problem, but it seems
most obvious to simply skip the rounding up if it would create too
many blocks in the AG.  Worst case, we lose 1 block per AG.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Alex Elder <aelder@sgi.com>